Abstract
This paper presents the results of uranium and thorium measurements made in aluminium and iron phosphates that occur in the Abrolhos Archipelago (offshore Brazil) by instrumental neutron activation analysis. The U concentrations were also indirectly determined from the gamma-ray activity of 234Th, 214Pb and 214Bi, while those of Th were obtained from the 228Ac, 212Pb, 212Bi and 208Tl gamma-ray emitters. In both cases it was assumed that 238U and 232Th series are in radioactive secular equilibrium. The comparison between the results obtained by neutron activation and gamma-ray spectrometry indicates that the 238U series is in disequilibrium in some of the investigated phosphates. The 234U/238U activity ratios determined by alpha spectrometry also indicate the disequilibrium in this series. For the 232Th decay chain the concentrations obtained by neutron activation and gamma-ray spectrometry show radioactive secular equilibrium in the analysed samples.
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